This invention relates generally to a read-only memory (ROM) of the type used to store permanent data and more particularly, to a double layer ROM where storage capacity is substantially increased without increasing chip size. A read-only memory is commonly utilized in many integrated circuits as a principle component. Generally, a ROM utilized in an integrated circuit is constructed by arranging transistors or diodes in a matrix on one plane. As a result, the ROM occupies a rather large area in the integrated circuit. Because formation of plural elements in overlapping relationship one to another in an integrated circuit has not been commonplace or has been impossible, the rather large area that a ROM occupies cannot be shared with another component. Heretofore, there have been only two ways to increase the capacity of a ROM, that is, to increase the pattern area for the ROM and consequently the chip area, or to miniaturize elements and increase the number of transistors in a given chip area. However, increasing the chip area leads to deterioration of the device and high manufacturing cost. Miniaturization of elements is limited by the technical capability at the present time. Therefore, means to enhance efficiency in using the pattern area for a ROM so as to increase the storage capacity has been greatly desired.
What is needed, is an integrated circuit ROM which stores more data per unit of chip area than has been conventionally achieved.